Some tubing is a bit harder than others which means a little extra force is needed to get it over the fittings. A very simple trick is to heat up a mug of water in the microwave. You don't want it boiling but you want it quite warm and dip the end of the tube in it letting it sit for a while (you need to experiment here a bit depending on how bad it is). Doing so will soften the ends making it much easier to slide over the fitting then let it cool.